1. Field of the Invention
The present invention relates to a method for molding plastic material by a mechanical press.
The principal material which is molded according to the method of this invention is plastic sheet material having a reinforcing agent such as glass fiber or the like mixed therein. The sheet material which is molded therefrom is comprised of glass fiber or the like impregnated with polypropylene or the like, and the material is such that the glass fiber will flow homogeneously during molding. The characteristics of the above material are that there can be molded therefrom shapes which are light in weight, are very strong, are corrosion resistant, have an excellent flow property, have little directionality, have excellent stability with respect to size, and have flexibility in design, making it possible to integrally mold complicated shapes.
In addition to being advantageous for molding the above plastic sheet material having a reinforcing agent, the method of the invention is advantageous for molding of plastic sheet material without any reinforcing agent therein, and the materials molded by this invention include both of these advantages.
2. Description of the Prior Art
In recent times, there has been a tendency to use plastic material mixed with a reinforcing agent therein as a substitute for metal, particularly for automobile parts, e.g. frames for the back seat of an automobile.
In the molding of plastic sheet, an oil pressure press (oilic press) which is easy to control, has been found to be quite suitable because of the need to maintain the molding pressure for a certain length of time after the final shape has been reached. This is due to the phenomenon that when the plastic sheet material is heated for use in pressure molding, the sheet material expands to a thickness several times the thickness of the raw sheet material. Therefore, if it is pressurized rapidly near the lower dead center position of the pressure member of the press, as in the case of a mechanical press, gas within the mold die cannot escape, and, as a result, the surface of the finished piece becomes rough or air bubbles are formed inside the piece by the gas remaining inside, thereby considerably lowering the commercial value of the product.
However, even when an oil pressure press is used, there are disadvantages due to the slow speed of the slide which expedites cooling of the material so that defective or imperfect products are produced or that it is difficult to mold tall or deep products. In order to avoid these disadvantages, increasing the temperature of the sheet material or the temperature of the dies has been tried, but this solution has resulted in deterioration of the material and has affected productivity because the cooling time during molding has to be made longer.